This invention relates to surface monitoring systems of the type that employ a three dimensional, hereinafter referred to as 3-D, non-destructive profilometry to directly measure the surface compression layer depth resulting from a surface treatment, of a substrate, such as, a shot peening treatment. Such systems of this type generally allow the compression layer depth to be directly measured without having to destruct the substrate, typically, by cutting the substrate using conventional cutting techniques such that the surface treatment operation can be monitored to determine if the desired compression layer depth is being achieved. In particular, lines are traced across a substrate which has been surface treated, for example, by shot peening and these line traces are transformed into 3-D surface profilograms which measure the intensity of the surface treatment. This invention relates to certain unique 3-D, non-destructive, surface treatment monitoring systems and monitoring means in association therewith.
During a surface treatment operation such as a shot peening process, a stream of shot (i.e., particles), travelling at a high velocity, is directed at a workpiece surface. The shot is directed at the workpiece so as to cause plastic deformation of the workpiece surface, which often is a metal surface. Although the process may be applied for other purposes, the shot peening process generally is used to increase fatigue strength of the workpiece.
For example, residual stress near the surface of high performance machine parts is directly related to the fatigue life of the part. If the surface is in a state of residual compression, the growth of microcracks created by, for example, hard processing, should be inhibited. Shot peening is a very effective means for producing surface compression residual stress, and therefore, prolonging the useful life of the part.
Determining the state of surface compression due to shot peening, however, has proven to be very difficult. There are currently several methods used to measure surface compression. In particular, there is a direct method for determining surface compression due to shot peening. Under this direct method the workpiece is cut apart by conventional cutting techniques, and then the depth, i.e. the thickness, of the surface compression is physically measured. This direct method is time consuming and requires destructing the part being analyzed. A more advantageous system, then, would be presented if such amounts of time and destruction were reduced.
Another known method for determining surface compression due to shot peening which is less time consuming and avoids the destruction of the workpiece is referred to as an indirect Almen method. In the Almen method, a strip of material is shot peened, and then the strip is analyzed to determine the surface compression due to the shot peening. The Almen method is indirect, in that, the effects of shot peening are not measured directly from a workpiece, rather, a substitute or Almen strip is utilized. However, the Almen strip method is subject to insensitivity due to process changes which may occur in the peening operations between Almen strip checks. Also, when peening workpieces having contoured surfaces, it is difficult to reproduce the peening conditions on the contour surfaces with an Almen strip which is usually flat. Finally, variations in the Almen strips themselves render the Almen strip method subject to error. Consequently, a still more advantageous system would be presented if such amounts of insensitivity, inapplicability and variations could be reduced while still avoiding the destruction of the workpiece.
Finally, there has been developed a method and system for monitoring shot peening which utilizes two-dimensional, hereinafter referred to as 2-D, line trace information. Exemplary of such a prior art system is U.S. patent application Ser. No. 473,781 to Thompson entitled "A Method and System for Monitoring Shot Peening" and assigned to the same assignee as the present invention. While this system has met with a degree of commercial success, the system is limited in that only a mere 2-D view of the surface treatment intensity can be obtained. Therefore, a further advantageous system, then, would be presented if a more complete analysis of the surface treatment could be presented.
It is apparent from the above that there exists a need in the art for a substrate surface treatment monitoring system which will not destruct the substrate in order to monitor the surface treatment, but which will monitor the surface treatment on the substrate surface in a manner which provides a full and complete analysis of the surface treatment through the use of 3-D profilometry. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.